1. Field of the Invention
The present invention relates to a signal transmitting apparatus for an orthogonal frequency division multiplexing (OFDM) system, and more particularly, to a signal transmitting apparatus which integrates companding and predistortion operations.
2. Description of the Related Art
The wireless local area network standard IEEE 802.11a/g/n adopts OFDM wireless communication as the standard for wireless communication systems. Generally, an OFDM system has two major problems when transmitting signals. The first problem is related to overlarge peak to average power ratio (PAPR) for transmitting signals, and the second problem is related to a nonlinear problem at the power amplifier.
PAPR is the ratio of peak signal strength to mean signal strength, and the probability of the signal transmitting at peak strength is much less than the probability of the signal transmitting at mean strength. Therefore, to limit the strength of the signal within the effective range of the system, the peak strength cannot exceed the maximum input strength of the system. If the system has an excessive PAPR value, the situation is such that most of the time the output of the signal is too small and the signal transmission efficiency is low.
In another aspect, power amplifiers can be divided into linear and non-linear types. The input to output ratio of the linear power amplifier is a constant, but the input to output ratio of the non-linear power amplifier is not a constant. Therefore, the non-linear power amplifier causes signal distortion. However, because the efficiency of outputting power for the linear power amplifier is far less than for the non-linear power amplifier, i.e., the power consumption of the linear power amplifier is greater than that of the non-linear power amplifier, normal systems usually adopt non-linear power amplifiers.
Traditional transmitting apparatuses of an OFDM system are composed of a baseband signal processing module and a radio frequency (RF) module. The baseband signal processing module is used to handle the PAPR problem, and the RF module is used to handle the non-linear problem of the power amplifier.
A normal solution to resolve the PAPR problem is to use a compandor, which compresses or expands transmitted signals at the transmitter so as to reduce PAPR of the signals. In the receiver an inverse compandor is used to restore the signals. In another respect, a normal solution to resolve the non-linear problem is to use a predistortion technique, which focuses on the input/output relationship of the power amplifier and conducts a predistortion operation on the transmitted signals so as to offset the non-linear effect. In the receiver an inverse predistortion operation is conducted to restore the signals.
Traditional baseband signal processing modules and RF modules handle these two problems separately. That is, these two modules strive for their respective best solutions, but ignore the best solution for the entire structure. Therefore, it is necessary to integrate these two modules in order to increase the efficiency of transmission and outputting power.